The present invention relates to a liquid-toner-development-type electrophotographic apparatus. More particularly, the invention relates to a liquid-toner-development-type electrophotographic apparatus in which a toner layer on an intermediate transfer member is melted through application of heat to separate a carrier solvent and a solid component from each other, and the carrier solvent is then removed.
1. Prior Art
A conventionally known liquid-toner-development-type electrophotographic apparatus uses as a liquid developer a highly viscous liquid toner composed of a liquid carrier (oil) and solid particles, such as pigment, which are dispersed in the liquid carrier (as disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 11-65290). A powder toner involves the following problems: toner particles scatter; and toner particles have a relatively large particle size of 7 xcexcm to 10 xcexcm, resulting in poor resolution. By contrast, a liquid toner has a small toner particle size of about 1 xcexcm and can hold a large amount of electrostatic charge. Thus, a toner image is unlikely to be disturbed, and high resolution can be achieved.
FIG. 4 shows the overall configuration of a conventional liquid-toner-development-type electrophotographic apparatus. In FIG. 4, a photosensitive drum 10 is electrostatically charged at about 700 V by means of a charger 11. Subsequently, the photosensitive drum 10 is exposed to light by means of an exposure unit 12, whereby an electrostatic latent image is formed such that an exposed portion assumes an electric potential of about 100 V. A prewetting unit 13 applies silicone oil having a viscosity of about 20 cSt to the surface of the photosensitive drum 10, to a thickness of 4 xcexcm to 5 xcexcm.
Developing units 14 corresponding to yellow, magenta, cyan, and black are provided and use as a liquid developer a nonvolatile toner of high viscosity and high concentration having a toner viscosity of 400 mPaxc2x7S to 4000 mPaxc2x7S and a carrier viscosity of 20 cSt. A developing roller supplies the liquid developer while being in contact with the photosensitive drum 10, in such a manner as to maintain a two-layer structure composed of a toner layer on the developing roller and a prewetting liquid film on the photosensitive drum 10, to thereby cause toner particles contained in the liquid developer to adhere to the photosensitive drum 10 according to an electric field established between the same and the photosensitive drum 10.
An intermediate transfer member 15 is biased at about xe2x88x92500 V to thereby transfer toner particles thereto from the photosensitive drum 10 in the order of yellow, magenta, cyan, and black according to an electric field established between the same and the photosensitive drum 10. A backup roller 20 is adapted to fix on printing paper the toner which is present on the intermediate transfer member 15 and is melted by means of a heating unit 18. The heating unit 18 heats a portion of the surface of the intermediate transfer member 15 at a position located upstream of the backup roller 20.
A carrier solvent used in liquid development is intended to prevent scattering of toner particles, which assume a particle size of about 1 xcexcm, as well as to uniformly disperse toner particles through electrification of the toner particles. In development and electrostatic transfer processes, the carrier solvent serves as a xe2x80x9cbridgexe2x80x9d to facilitate movement of toner particles, which is effected by means of electric-field action.
In a liquid-development electrophotographic process, the carrier solvent is a component necessary for storage of toner, transport of toner, formation of a toner layer, and electrostatic transfer of toner. However, during and after the step of fixation of toner on paper medium, the carrier solvent is a component unnecessary for obtainment of good picture quality. Thus, at present, a volatile insulating liquid is used as a carrier solvent in many liquid developers (liquid toners). However, in consideration of fixation of toner within apparatus due to volatilization of a carrier, and effects of a volatile carrier on the human body and the environment, an electrophotographic apparatus which uses a liquid developer using a nonvolatile carrier solvent; for example, an HVS (High-Viscosity Silicone) toner, as shown in FIG. 4, is developed.
In some cases, a liquid-development toner using a nonvolatile carrier solvent may involve the following problem: the carrier solvent cannot be volatilized during melting of toner through application of heat to the liquid toner and, particularly during fixation of toner or thermal transfer of a toner image, hinders development of adhesion of a molten toner onto a paper medium, resulting in a failure to attain satisfactory picture quality and fixation strength with respect to a toner image transferred onto the paper medium.
As mentioned previously, the intermediate transfer member 15 must be heated by appropriate heating means located outside or inside the same. However, this configuration involves the following problems: since the intermediate transfer member 15 is heated at all times, the photosensitive drum 10 is heated, with a resultant deterioration in photosensitive properties; since, during removal of a carrier, a toner image is in a molten state through exposure to heat, the toner image is disturbed due to adhesion to a carrier-removing roller, resulting in an impairment in picture quality; and in a process of superposing toner images, a toner image which has previously been electrostatically transferred is in a molten state through exposure to heat and is thus disturbed during contact with the photosensitive drum, resulting in an impairment in picture quality.
2. Related Art
To solve the above problems, the present applicant filed with the Japanese Patent Office a patent application for a liquid-toner-development-type electrophotographic apparatus comprising a cooling roller for eliminating influence of heating of an intermediate transfer member on a photosensitive drum and means for removing excessive carrier at a position located upstream of a heating position where a toner layer on the intermediate transfer member is melted through application of heat (Japanese Patent Application No. 11-26960 filed on Feb. 4, 1999).
FIG. 3 shows heating and cooling mechanisms proposed in the above filed application. In FIG. 3, toner particles transferred from a photosensitive drum 10 to an intermediate transfer belt 24 are melted through application of heat. The molten toner is transferred onto a printing medium, such as paper, in a single operation. At this time, a backup roller 20 abuts a heating roller 25 under pressure so as to fix the molten toner onto the medium, such as printing paper.
The intermediate transfer belt 24 is looped around a plurality of tension rollers 26 and 27, the heating roller 25, and the cooling roller 23. A carrier-removing roller 21 is disposed in contact with the surface of the intermediate transfer belt 24 at a position located upstream of a position where the intermediate transfer belt 24 comes into contact with the heating roller 25. A bias potential is applied to the carrier-removing roller 21 while a conductive roller 19 located in opposition to the heating roller 25 is grounded, thereby removing excessive carrier as well as prewetting liquid without exerting an electrical influence on other processes, such as electrostatic transfer and thermal transfer. However, excessive carrier solvent cannot be completely removed.
Various proposals have been put forth for removing as much carrier solvent as possible from a toner image on the photosensitive drum or the intermediate transfer member. However, in the case of a nonvolatile oil, removing almost all nonvolatile oil trapped between toner particles is nearly impossible.
When toner particles, or a solid component of toner, are melted and integrated through application of heat, oil trapped between toner particles is isolated. Through utilization of this phenomenon, oil removal can be performed effectively.
An object of the present invention is to provide a liquid-toner-development-type electrophotographic apparatus in which the nonvolatile carrier solvent remains in a toner image on an intermediate transfer member and in which residual carrier solvent which is not removed in a carrier-removing process conducted before a heating process can be efficiently removed through utilization of a phenomenon that toner particles (resin component) are melted and integrated in the heating process and a phenomenon that a carrier solvent (liquid component) is isolated in the heating process.
In the liquid-toner-development-type electrophotographic apparatus of the present invention, a toner image is transferred from a photosensitive member onto an intermediate transfer member, and toner particles transferred onto the intermediate transfer member are melted through application of heat and transferred onto a printing medium. A carrier-removing roller, which serves as means for removing a carrier solvent, is disposed downstream of a position where a toner layer on the intermediate transfer member is melted through application of heat, and upstream of a position where the intermediate transfer member again abuts the photosensitive member.
Also, there is disposed a carrier-removing roller, which serves as means for removing the carrier solvent from the intermediate transfer member while the toner layer on the intermediate transfer member is being melted through application of heat, at a position where the toner layer is melted through application of heat.
The present invention is characterized by comprising at lease either the means for removing the carrier solvent after the toner layer is melted through application of heat, or the means for removing the carrier solvent while the toner layer is melted through application of heat. However, the present invention may comprise both of the means and may further comprise means for removing the carrier solvent (a carrier-removing roller) disposed upstream of the position where the toner layer on the intermediate transfer member is heated.
Thus, the liquid-toner-development-type electrophotographic apparatus of the present invention can separate the carrier solvent and the solid component to thereby remove the carrier solvent, by melting, through application of heat, the toner layer which is formed on the intermediate transfer member and contains a nonvolatile oil component trapped between toner particles.